In order to identify a substance with a large molecular weight and analyze the structure of the substance, an MS/MS analysis (or tandem analysis) is known as one of the mass spectrometric methods. In MS/MS analysis, an ion having a specific mass-to-charge ratio among various ions produced from a sample is selected as a precursor ion (which is the first stage mass separation), the precursor ion is dissociated by some method including one that brings the precursor ion into contact with a CID gas, and various product ions produced by the dissociation are separated according to the mass-to-charge ratios (which is the second stage mass separation) before they are detected.
A triple quadrupole mass spectrometer in which a collision cell is disposed between the quadrupole mass filters at a front stage and at a rear stage is a type of mass spectrometer capable of MS/MS analysis having a relatively simple structure which is widely used. Another configuration of a mass spectrometer is known in which the rear stage quadrupole mass filter of the triple quadrupole mass spectrometer is replaced with a time-of-flight mass spectrometer which has a higher mass resolution (see Patent Document 1, etc.). In the present description, a mass spectrometer that carries out two-stage mass separation as described above is called a tandem mass spectrometer. It is also called an MS/MS mass spectrometer.
In general, the dissociating pattern of a compound by CID or the like is not unique, and the same compound show different dissociating patterns depending on the CID conditions such as the magnitude of the collision energy given to the ions at the time of CID, and the gas pressure in the collision cell. This is because various bonding sites in a compound can be cut depending on the CID conditions. The main information that is obtained by the mass spectrum of MS/MS analysis is the information of masses of various fragments that are generated as the result of dissociation of the precursor ion derived from the target compound. Accordingly, in order to estimate the molecular structure of the target compound, it is more favorable if the mass information of a larger variety of fragments derived from the compound is obtained.
As previously described, in a tandem mass spectrometer, the dissociating pattern can be changed by changing the CID condition. Therefore, in the mass spectrometer described in Patent Document 1, MS/MS analyzes to the same sample are executed under the CID condition in which dissociation easily occurs and under the CID condition in which relatively less dissociation occurs, respectively, so that a highly fragmented mass spectrum and a less fragmented mass spectrum are acquired. In this case, the analyzer obtains more information by comparing both the mass spectra, for example, than in the case of simply using an Ms/MS spectrum under one CID condition, and can increase the estimating reliability of the structure of the target compound.
However, in the mass spectrometer described in Patent Document 1, only two kinds of information of the highly fragmented mass spectrum and the less fragmented mass spectrum can be obtained, and the mass spectrometer is not always sufficient for analyzing the structure of a compound having a complicated molecular structure. Though it is possible to modify the mass spectrometer described in Document 1 to acquire three or more MS/MS spectra with different CID conditions, it takes some time to perform a mass spectrometry over a certain range of mass-to-charge ratio, and therefore, the time needed to obtain a number of MS/MS spectra to one compound under different CID conditions becomes long correspondingly.
Especially when a gas chromatograph (GC) and a liquid chromatograph (LC) are connected to the front stage of the mass spectrometer, and compounds temporally separated by the chromatographs are analyzed by the mass spectrometer, the time width in which one compound is introduced in the mass spectrometer is significantly limited. Therefore, if the time required for analyzing one compound becomes long, the analysis will not finish in the time period in which the compound is introduced in the mass spectrometer. That is, the objective ions derived from the compound to be analyzed will be totally consumed before a plurality of mass spectra for the compound are fully obtained.